1. Field of the Invention
The present invention relates to a glass lens blank for polishing, a method of manufacturing the same, and a method of manufacturing an optical lens.
2. Description of Related Art
As a method of manufacturing a lens, the following method has been known. That is, square-shaped or plate-shaped glass blocks are prepared by flowing a molten glass into a mold, and then these glass blocks are cut and subdivided by a mechanical processing to thereby produce cut pieces. Subsequently, together with equalizing the weight of each of the cut pieces, a rough-polishing processing (barrel polishing) is performed on the surfaces thereof so as to facilitate adhering a release agent. Thereafter, the rough-polished cut pieces are softened by reheating, and the softened glass is press-molded to thereby mold a lens blank having a shape approximating to a lens shape. Finally, the lens blank is ground and polished to manufacture a lens.
According to this method, if plate-shaped glass blocks of a plurality of glass types are prepared and stored, it is possible to cut the plate-shaped glass block to a desired quantity and volume as necessary and press the cut pieces thereof. Accordingly, the method is suitable for small amount of a wide variety products. However, in this method, on a surface of the lens blank after press-processing the cut pieces, a defect-containing layer is formed in a thickness of 60˜500 μm. The defect-containing layer includes portions in which glass component is altered due to contacts with cracks occurred during the rough-polishing processing or a processing solution, or a crystallized portion generated from the time of softening to the time of press-molding. Furthermore, when softening the cut pieces, a phenomenon is caused that each corner is folded into inside the glass, and its folded amount reaches to 300 μm or more, such that this sometimes occurs in a deep portion as the defect-containing layer.
If such a defect-containing layer (including folded portions) is formed on a surface of the glass blank, when in a subsequent step, trying to manufacture a lens having a smooth surface without irregularities together with removing the defect-containing layer, it is necessary to perform grinding and polishing of 500 μm or more, and as a result unnecessary use of materials is generated together with grinding and polishing time being longer.
Recently, as disclosed in Patent Document 1 (Japanese Patent No. 3806288) below, there has been also proposed a method of manufacturing a glass lens blank for polishing by steps as follows. That is, this method comprises a step of receiving and molding a molten glass supplied to be from a nozzle with a mold to obtain a glass gob, a step of rough-polishing processing a surface of the glass gob, a step of forming a powderary release agent on the surface of the glass gob, and a step of reheating and press-molding the glass gob.
In this method, since occurrence of angulated portions such as the cut pieces may be suppressed on the surface of the glass gob, it is possible to perform a molding without causing the folded portions in the next reheating step. However, also in this method, a defect-containing layer is formed in a thickness of the order of 60˜150 μm as the above. Further, in the fact of being capable of manufacturing a number of glass gobs and storing them as a stock, this method enables a wide variety productions by press-molding a plurality of glass gobs having a same shape with a plurality of molds having different molding surfaces upon an order.
In any conventional methods, since in the case of grinding and polishing the glass lens blank for polishing after press-molding into the lens, the processing time required for grinding and polishing occupies about half of the entire process for an optical lens, a reduction in the processing time has been demanded. Further, if the thickness of the defect-containing layer on the surface of the glass lens blank for polishing is reduced, it is expected that it is possible to reduce the processing time required for grinding and polishing; however, in the conventional methods, a glass lens blank for polishing having a thickness of the defect-containing layer of 50 μm or less has not been achieved. In particular, as for a glass lens blank for polishing that is a basis for a medium- or large-diameter lens to be used in a single-lens reflex or the like, in the conventional methods, it has been considered that it is impossible to manufacture a thin glass lens blank for polishing having a thickness of the defect-containing layer of 50 μm or less.